1. Field of the Invention
This invention relates to a semiconductor light-emitting device and a method of manufacturing the same.
2. Description of the Related Art
A CSP (channeled substrate planar) laser, TJS (transverse junction stripe) laser, PBR (polyimide buried ridge) laser and the like are examples of current-restriction-type semiconductor light-emitting devices. The cost and device performance of such lasers are decided by the simplicity and reliability of the manufacturing process.
An example of the structure of a CSP laser is illustrated in FIG. 3. This CSP laser is fabricated by the following manufacturing process:
A stripe-shaped groove 26 having a width of about 5 .mu.m is formed in an n-GaAs substrate 21, and an n-Al.sub.0.33 Ga.sub.0.67 As layer 22 is grown on the substrate so as to smoothen and flatten the entire surface thereof. Next, an Al.sub.0.05 Ga.sub.0.95 As active layer 23, a p-Al.sub.0.33 Ga.sub.0.67 As layer 24 and an n GaAs layer 25 are successively grown. Thereafter, Zn diffusion is performed at a position corresponding to the stripe-shaped groove 26 (the Zn-diffusion region is indicated at numeral 27) to produce a current-pinched region. Finally, the top and bottom sides of the element are provided with a p-electrode 28 and an n-electrode 29, respectively.
The CSP laser is manufactured by an LP process (liquid-phase epitaxy) or MOCVD process (metal-organic chemical vapor deposition).
This CSP laser and its method of manufacture have the following drawbacks:
1) In order to obtain high-quality crystal growth with the LP process, the entire surface of the substrate is etched lightly (referred to as "light etching") prior to crystal growth. However, since the substrate of the CSP laser is processed to have the groove of the about 5 .mu.m width before crystal growth takes place, the corners of the groove will be blunted if the substrate surface is subjected to light etching. For this reason, the light etching step is omitted. As a consequence, crystal growth takes place with the substrate surface left unsmoothened, as a result of which crystals of high quality cannot be obtained.
2) As mentioned above, only the LP process or MOCVD process can be used to grow crystals on a substrate having a groove. However, when it is attempted to grow crystals on a substrate surface having a groove by either of these methods, control is difficult to perform since the rate of growth is dependent upon the surface orientation, back etching occurs and the shape of the groove itself changes.
3) Since it is necessary to register a mask accurately with respect to the groove position in the Zn-diffusion step, the process is a complicated one.
4) The Zn-diffusion step itself requires accurate temperature control and a long period of time.
In view of the foregoing, problems are encountered in terms of reducing cost and achieving performance stability.
An example of the structure of a TJS laser is depicted in FIG. 4. The process for manufacturing such a TJS laser is as follows:
An n-Al.sub.0.4 Ga.sub.0.6 As layer 32, n.sup.+ -Al.sub.y Ga.sub.1-y As active layer 33, an n-Al.sub.0.4 Ga.sub.0.6 As layer 34, a p-Al.sub.0.4 Ga.sub.0.6 As layer 35 and an n.sup.- -GaAs layer 36 are successively grown on an n-GaAs substrate 31. Thereafter, p.sup.+ -Zn diffusion is performed at a predetermined region 37, and finally a p-electrode 39 and n-electrode 40 are provided. Current injected from the p.sup.+ -Zn diffusion region 37 flows laterally through the active layer 33, as indicated by the arrow, and this forms a type of current-pinched region as well as an oscillation region 38 in which lasing taking place.
Since the structure of the TJS laser is such that crystals are caused to grow on a flat substrate, crystal growth itself takes place with facility but the manufacturing process has the following shortcomings:
5) The Zn diffusion step requires accurate temperature control and a long period of time.
6) Since Zn diffusion also occurs in the lateral direction, the oscillation region lacks positional controllability.
In view of the foregoing, problems are encountered in terms of reducing cost and achieving performance stability.
The applicant has proposed a PBR laser [for example, see Japanese Patent Application Laid-Open (KOKAI) No. 63-122187] the manufacturing process of which will now be described with reference to FIGS. 5a through 5e. First, an n-Al.sub.x Ga.sub.1-x As cladding layer 42, a GaAs active layer 43, a p-Al.sub.x Ga.sub.1-x As cladding layer 44 and a p.sup.+ -GaAs cap layer 45 are grown on an n-GaAs substrate 41 by a single process, and a stripe-shaped etching mask 47 having a width of about 5 .mu.m is formed on the cap layer 45 as by a photoresist (FIG. 5a). Next, in FIG. 5b, portions of the cladding layer 44 and cap layer 45 not covered with the mask 47 are etched down to an intermediate level of the cladding layer 44 (by chemical etching or dry etching), thereby forming a ridge portion. Further, in FIG. 5c, the etching mask 47 is removed, after which the entire surface of the semiconductor wafer is coated with enough heat-resistant polyimide resin 48 to flatten the top surface. The resin is allowed to harden. Thereafter, in FIG. 5d, the resin 48 is removed by ashing using oxygen plasma until the crown of the ridge portion is reached. Finally, a p-side electrode 49 and an n-side electrode 50 are vapor-deposited on the top and bottom portions of the element, respectively, as shown in FIG. 5e.
The cap layer 45 and the top surface of the resin 48 are flush and flat, and the electrode 49 is formed over the entirety of these upper surfaces. The low, flat portions on both sides of the ridge are buried by the resin 48.
When a current is passed through the thus-obtained PBR semiconductor laser from the electrode 49 to the electrode 50, no current flows into the low flat portions on both sides of the ridge owing to the resin 48, and current concentrates only in the ridge portion. As a result, only the portion of the active layer 43 underlying the ridge serves as a light-emitting portion and performs transverse-mode controlled lasing.
Since this PBR laser is such that the growth and etching process both need be performed only one time, it is possible to lower cost.
In addition, owing to crystal growth on a flat substrate and the use of an etching process having excellent controllability, the device performance exhibits a high stability. These are the characterizing features of this laser.
Furthermore, since the top surface formed to have the ridge is flat, so-called junction-down mounting (a mounting method exhibiting an excellent heat-dissipating characteristic in which the element is turned upside down so that the substrate 41 is on top,) becomes possible and a higher output can be expected.
However, though the polyimide resin used for burying the ridge portion possesses excellent characteristics in terms of electrical insulation and thermal expansion, its thermal conductivity is poor and therefore the heat produced in the oscillation region cannot be given off efficiently to the outside. Even if the abovementioned junction-down method is employed, there is little increase in the efficiency of thermal dissipation and the service lifetime of the device is hardly improved. In addition, since the height of the ridge is on the order of 2 .mu.m, there is the danger of a short circuit when an electrically conductive adhesive containing In, Sn or the like is used in junction-down mounting.